1. Field of the Invention
This invention relates to integrated circuit (IC) technology, and more particularly, to a universal crystal-oscillator input/output (I/O) circuit for use with an ASIC (Application Specific Integrated Circuit) device, and which can hell) enhance the ESD (Electrostatic Discharge) protection on the ASIC device in Charge Device Mode (CDM).
2. Description of Related Art
In the fabrication of ICs, electrostatic electricity would easily cause damage to fabricated IC devices. It can occur at many places in the factory, such as the body of an operator and the moving parts of factory installations. When electrostatic electricity comes into touch with an IC device, it can cause electrostatic, discharge (ESD) to the IC device in various modes, such as Human Body Mode (HBM), Charge Device Mode (CDM), and Machine Mode (MM). The ESD would easily cause damage to the IC device.
MOSFET (metal-oxide-semiconductor field-effect transistor) based IC devices are particularly vulnerable to ESD due to the high input impedance and low breakdown voltage of the MOSFET. One solution to this problem is to provide an ESD protection circuit at the I/O ports of the MOSFET-based IC device.
Crystal Oscillator Pad is an important component in ASIC (Application Specific Integrated Circuit) library, which provides a high quality (Q) factor that allows the output oscillator frequency to be highly stable. With advances in IC technology, however, a single ASIC library now can contain varieties of Crystal Oscillator Pad components, which causes inconvenience to the ASIC library vendors and ASIC designers.
FIG. 1 is a schematic circuit diagram of a conventional CMOS (complementary metal-oxide-semiconductor) based crystal-oscillator I/O circuit. As shown, this CMOS-based crystal-oscillator I/O circuit includes a PMOS (P-type metal-oxide-semiconductor) transistor 12, an NMOS (N-type metal-oxide-semiconductor) transistor 14, a feedback resistor 16, and a crystal 18. The PMOS transistor 12 and the NMOS transistor 14 in combination constitute a CMOS transistor, as designated by the reference numeral 15. This CMOS transistor 15 serves as an inverter and an amplifier. Moreover, the CMOS transistor 15 is mounted on the inside of an ASIC, as the part enclosed by the dashed box indicated by the reference numeral 10. The feedback resistor 16 is used to provide a suitable DC operating point that can provide the required bias to the CMOS transistor 15. The crystal 18 is used to limit the output frequency of the crystal oscillator.
Conventionally, a ASIC library is provided with a variety of crystal-oscillator I/O components for the designer to select. However, this would increase the implementation cost of the ASIC library. Moreover, these crystal-oscillator I/O components are all unable to provide the optimal device transconductance for various kinds of voltage sources and oscillation frequency, and therefore, would not satisfy different customer requirements. For instance, some customers would request a voltage down conversion from, for example, 3.3 V to 2.4 V or 1.5 V.
Moreover, CDM ESD protection on VLSI can easily fail. From actual CDM ESD test, it was found that the IO circuit would be incapable of ineffectively drain the electrostatic electricity accumulated on the IC substrate without CDM ESD protection circuit, thus resulting in a poor protection capability to the oscillator I/O circuit. In consequence, he oscillator I/O circuit would be easily damaged by ESD.